Description (applicant's description): The first step in vision is the interaction of light with rhodopsin, a protein bound to a light absorbing chromophore, 11-cis retinal. Photo absorption causes retinal to convert from a cis to trans conformational change, and this change in structure is transmitted to the surrounding protein. A series of protein-protein interactions are altered by the structural change, and a G-protein cascade is induced, eventually leading to a nerve impulse. This process is very well studied and its role in human perception and disease is understood at the basic level. However, the three-dimensional, atomic level structure of rhodopsin is not yet known. This is because the protein is a transmembrane receptor. Membrane proteins are very difficult to study using NMR or X-ray crystallography, the latter because of difficulties in obtaining crystals. Recently, crystals of bovine rhodopsin have been generated that are the focus of this proposal. X-ray crystal structure analysis has been initiated using these crystals, and a low resolution structure is available confirming the presence of seven transmembrane helices. In this project, more ordered crystals will be grown to provide a higher resolution view of the structure, including the inter-membrane polypeptide loops important for interactions with G-proteins. Trapped photostates of rhodopsin, several of its derivatives with altered chromophores, and mutants for the protein will also be studied crystallographically. These three-dimensional molecular structures will be used to correlate the biochemical and biophysical information about rhodopsin and its interactions with other molecules. In addition, this first structure of a G-protein coupled receptor will provide a base for homology modeling of other members of this pharmaceutically important protein family.